Heretofore, piezoelectric ceramic compositions consisting essentially of lead titanate zirconate (Pb(Ti.sub.x Zr.sub.1-x)O.sub.3 with 0&lt;x&lt;1) or lead titanate (PbTiO.sub.3) have been widely used for piezoelectric ceramic devices such as piezoelectric ceramic sensors. However, those piezoelectric ceramic compositions lose their piezoelectricity at temperatures above their Curie point, which is between 200 and 500.degree. C. or so. Therefore, they could not be used as materials for piezoelectric ceramic sensors acting at temperatures above 500.degree. C.
Compared with lead titanate zirconate and lead titanate, the Curie point of the layer-structured bismuth compound Na.sub.0.5 Bi.sub.4.5 Ti.sub.4 O.sub.15 (hereinafter referred to as NBT) has a high value of about 670.degree. C., and there is a hope of NBT piezoelectric materials usable at high temperatures. Other compositions derived from NBT by substituting a part of the moiety of NBT, (Na.sub.0.5 Bi.sub.0.5), with any of Ca, Sr, Ba, Pb, (K.sub.0.5 Bi.sub.0.5) or the like are also known as piezoelectric materials having a high Curie point.
However, piezoelectric ceramic compositions consisting essentially of such a layer-structured bismuth compound generally have a small piezoelectric constant, d.sub.33, compared with those consisting essentially of lead titanate zirconate or lead titanate, and are therefore unsatisfactory for piezoelectric ceramic sensors that act on the basis of d.sub.33, such as accelerometer sensors, etc.
Techniques of adding manganese to NBT or NBT-derived compositions produced by substituting a part of the NBT moiety, (Na.sub.0.5 Bi.sub.0.5), with Ca (hereinafter referred to as NBCT) have been disclosed in a report "Piezoelectricity in Ceramics of Ferroelectric Bismuth Compound with Layer Structure" (by S. Ikegami and I. Ueda, in Japanese Journal of Applied Physics, 1974) and in a report of "Production of Grain-oriented Ceramics, Their Characteristic, and Their Applications to Electronic Materials" (by T. Takenaka and K. Sakata, in Ceramics, published by Ceramics Association of Japan, 1989). Ikegami et al. added manganese in the form of MnO.sub.2 to NBT in an amount of 5 mol % of MnO.sub.2 (that is, in an amount of about 0.42% by weight in terms of MnCO.sub.3). However, they say nothing about any improvement in the piezoelectric characteristics of the compositions produced. Takenaka et al. added manganese to NBT and NBCT in an amount of from about 0.1 to 0.2% by weight in terms of MnCO.sub.3. However, the composition of NBT to which was added 0.1% by weight of MnCO.sub.3 has d.sub.33 of 15.6 pC/N, and that of NBCT to which was added 0.2% by weight of MnCO.sub.3 has d.sub.33 of 20.8 pC/N. Thus, d.sub.33 of the compositions of Takenaka et al. is still lower than the 25 pC/N which piezoelectric ceramic sensors require for practical application.